Early antiretroviral therapy in SIV-infected rhesus macaques reveals a multiphasic, saturable dynamic accumulation of the rebound competent viral reservoir

The rebound competent viral reservoir (RCVR)-virus that persists during antiretroviral treatment (ART) and can reignite systemic infection when treatment is stopped-is the primary barrier to eradicating HIV. We used time to initiation of ART during primary infection of rhesus macaques (RMs) after intravenous challenge with barcoded SIVmac239 as a means to elucidate the dynamics of RCVR establishment in groups of RMs by creating a multi-log range of pre-ART viral loads and then assessed viral time-to-rebound and reactivation rates resulting from the discontinuation of ART after one year. RMs started on ART on days 3, 4, 5, 6, 7, 9 or 12 post-infection showed a nearly 10-fold difference in pre-ART viral measurements for successive ART-initiation timepoints. Only 1 of 8 RMs initiating ART on days 3 and 4 rebounded after ART interruption despite measurable pre-ART plasma viremia. Rebounding plasma from the 1 rebounding RM contained only a single barcode lineage detected at day 50 post-ART. All RMs starting ART on days 5 and 6 rebounded between 14- and 50-days post-ART with 1-2 rebounding variants each. RMs starting ART on days 7, 9, and 12 had similar time-to-measurable plasma rebound kinetics despite multiple log differences in pre-ART plasma viral load (pVL), with all RMs rebounding between 7- and 16-days post-ART with 3-28 rebounding lineages. Calculated reactivation rates per pre-ART pVL were highest for RMs starting ART on days 5, 6, and 7 after which the rate of accumulation of the RCVR markedly decreased for RMs treated on days 9 and 12, consistent with multiphasic establishment and near saturation of the RCVR within 2 weeks post infection. Taken together, these data highlight the heterogeneity of the RCVR between RMs, the stochastic establishment of the very early RCVR, and the saturability of the RCVR prior to peak viral infection.

Myeloid cell tropism enables MHC-E-restricted CD8+ T cell priming and vaccine efficacy by the RhCMV/SIV vaccine

The strain 68-1 rhesus cytomegalovirus (RhCMV)-based vaccine for simian immunodeficiency virus (SIV) can stringently protect rhesus macaques (RMs) from SIV challenge by arresting viral replication early in primary infection. This vaccine elicits unconventional SIV-specific CD8+ T cells that recognize epitopes presented by major histocompatibility complex (MHC)-II and MHC-E instead of MHC-Ia. Although RhCMV/SIV vaccines based on strains that only elicit MHC-II- and/or MHC-Ia-restricted CD8+ T cells do not protect against SIV, it remains unclear whether MHC-E-restricted T cells are directly responsible for protection and whether these responses can be separated from the MHC-II-restricted component. Using host microRNA (miR)-mediated vector tropism restriction, we show that the priming of MHC-II and MHC-E epitope-targeted responses depended on vector infection of different nonoverlapping cell types in RMs. Selective inhibition of RhCMV infection in myeloid cells with miR-142-mediated tropism restriction eliminated MHC-E epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-II epitope-targeted response. Inhibition with the endothelial cell-selective miR-126 eliminated MHC-II epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-E epitope-targeted response. Dual miR-142 + miR-126-mediated tropism restriction reverted CD8+ T cell responses back to conventional MHC-Ia epitope targeting. Although the magnitude and differentiation state of these CD8+ T cell responses were generally similar, only the vectors programmed to elicit MHC-E-restricted CD8+ T cell responses provided protection against SIV challenge, directly demonstrating the essential role of these responses in RhCMV/SIV vaccine efficacy.

Interleukin-15 response signature predicts RhCMV/SIV vaccine efficacy

Simian immunodeficiency virus (SIV) challenge of rhesus macaques (RMs) vaccinated with strain 68–1 Rhesus Cytomegalovirus (RhCMV) vectors expressing SIV proteins (RhCMV/SIV) results in a binary outcome: stringent control and subsequent clearance of highly pathogenic SIV in ~55% of vaccinated RMs with no protection in the remaining 45%. Although previous work indicates that unconventionally restricted, SIV-specific, effector-memory (EM)-biased CD8⁺ T cell responses are necessary for efficacy, the magnitude of these responses does not predict efficacy, and the basis of protection vs. non-protection in 68–1 RhCMV/SIV vector-vaccinated RMs has not been elucidated. Here, we report that 68–1 RhCMV/SIV vector administration strikingly alters the whole blood transcriptome of vaccinated RMs, with the sustained induction of specific immune-related pathways, including immune cell, toll-like receptor (TLR), inflammasome/cell death, and interleukin-15 (IL-15) signaling, significantly correlating with subsequent vaccine efficacy. Treatment of a separate RM cohort with IL-15 confirmed the central involvement of this cytokine in the protection signature, linking the major innate and adaptive immune gene expression networks that correlate with RhCMV/SIV vaccine efficacy. This change-from-baseline IL-15 response signature was also demonstrated to significantly correlate with vaccine efficacy in an independent validation cohort of vaccinated and challenged RMs. The differential IL-15 gene set response to vaccination strongly correlated with the pre-vaccination activity of this pathway, with reduced baseline expression of IL-15 response genes significantly correlating with higher vaccine-induced induction of IL-15 signaling and subsequent vaccine protection, suggesting that a robust de novo vaccine-induced IL-15 signaling response is needed to program vaccine efficacy. Thus, the RhCMV/SIV vaccine imparts a coordinated and persistent induction of innate and adaptive immune pathways featuring IL-15, a known regulator of CD8⁺ T cell function, that support the ability of vaccine-elicited unconventionally restricted CD8⁺ T cells to mediate protection against SIV challenge.

SIV insert-expressing vaccine vectors based on strain 68–1 RhCMV elicit robust, highly effector-memory-biased, unconventionally restricted T cell responses that are associated with an unprecedented level of SIV control after challenge (replication arrest leading to clearance) in slightly over half of vaccinated monkeys. Since efficacy among monkeys vaccinated with the effective 68–1 vaccine is not predicted by standard measures of immunogenicity, we used functional genomics analysis of RhCMV/SIV vaccinated monkeys with known challenge outcomes to identify immune correlates of protection. We found that vaccine efficacy significantly correlates with a vaccine-induced response to IL-15 that includes modulation of immune cell, inflammation, TLR signaling, and cell death programming response pathways. These data suggest that RhCMV/SIV efficacy results from a coordinated and sustained vaccine-mediated induction of innate and adaptive immune pathways featuring IL-15, a known regulator of CD8⁺ effector-memory T cell function, that cooperates with vaccine-elicited CD8⁺ T cells to mediate efficacy.

Modulation of MHC-E transport by viral decoy ligands is required for RhCMV/SIV vaccine efficacy

Strain 68-1 rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) antigens elicit CD8+ T cells recognizing epitopes presented by major histocompatibility complex II (MHC-II) and MHC-E but not MHC-Ia. These immune responses mediate replication arrest of SIV in 50 to 60% of monkeys. We show that the peptide VMAPRTLLL (VL9) embedded within the RhCMV protein Rh67 promotes intracellular MHC-E transport and recognition of RhCMV-infected fibroblasts by MHC-E-restricted CD8+ T cells. Deletion or mutation of viral VL9 abrogated MHC-E-restricted CD8+ T cell priming, resulting in CD8+ T cell responses exclusively targeting MHC-II-restricted epitopes. These responses were comparable in magnitude and differentiation to responses elicited by 68-1 vectors but did not protect against SIV. Thus, Rh67-enabled direct priming of MHC-E-restricted T cells is crucial for RhCMV/SIV vaccine efficacy.

Cytomegaloviral determinants of CD8+ T cell programming and RhCMV/SIV vaccine efficacy

Simian immunodeficiency virus (SIV) insert-expressing, 68-1 rhesus cytomegalovirus (RhCMV/SIV) vectors elicit major histocompatibility complex E (MHC-E)- and MHC-II-restricted, SIV-specific CD8+ T cell responses, but the basis of these unconventional responses and their contribution to demonstrated vaccine efficacy against SIV challenge in the rhesus monkeys (RMs) have not been characterized. We show that these unconventional responses resulted from a chance genetic rearrangement in 68-1 RhCMV that abrogated the function of eight distinct immunomodulatory gene products encoded in two RhCMV genomic regions (Rh157.5/Rh157.4 and Rh158-161), revealing three patterns of unconventional response inhibition. Differential repair of these genes with either RhCMV-derived or orthologous human CMV (HCMV)-derived sequences (UL128/UL130; UL146/UL147) leads to either of two distinct CD8+ T cell response types-MHC-Ia-restricted only or a mix of MHC-II- and MHC-Ia-restricted CD8+ T cells. Response magnitude and functional differentiation are similar to RhCMV 68-1, but neither alternative response type mediated protection against SIV challenge. These findings implicate MHC-E-restricted CD8+ T cell responses as mediators of anti-SIV efficacy and indicate that translation of RhCMV/SIV vector efficacy to humans will likely require deletion of all genes that inhibit these responses from the HCMV/HIV vector.

Antibody-mediated depletion of viral reservoirs is limited in SIV-infected macaques treated early with antiretroviral therapy

The effectiveness of virus-specific strategies, including administered HIV-specific mAbs, to target cells that persistently harbor latent, rebound-competent HIV genomes during combination antiretroviral therapy (cART) has been limited by inefficient induction of viral protein expression. To examine antibody-mediated viral reservoir targeting without a need for viral induction, we used an anti-CD4 mAb to deplete both infected and uninfected CD4+ T cells. Ten rhesus macaques infected with barcoded SIVmac239M received cART for 93 weeks starting 4 days after infection. During cART, 5 animals received 5 to 6 anti-CD4 antibody administrations and CD4+ T cell populations were then allowed 1 year on cART to recover. Despite profound CD4+ T cell depletion in blood and lymph nodes, time to viral rebound following cART cessation was not significantly delayed in anti-CD4-treated animals compared with controls. Viral reactivation rates, determined based on rebounding SIVmac239M clonotype proportions, also were not significantly different in CD4-depleted animals. Notably, antibody-mediated depletion was limited in rectal tissue and negligible in lymphoid follicles. These results suggest that, even if robust viral reactivation can be achieved, antibody-mediated viral reservoir depletion may be limited in key tissue sites.

A live-attenuated RhCMV/SIV vaccine shows long-term efficacy against heterologous SIV challenge

Previous studies have established that strain 68-1-derived rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) proteins (RhCMV/SIV) are able to elicit and maintain cellular immune responses that provide protection against mucosal challenge of highly pathogenic SIV in rhesus monkeys (RMs). However, these efficacious RhCMV/SIV vectors were replication and spread competent and therefore have the potential to cause disease in immunocompromised subjects. To develop a safer CMV-based vaccine for clinical use, we attenuated 68-1 RhCMV/SIV vectors by deletion of the Rh110 gene encoding the pp71 tegument protein (ΔRh110), allowing for suppression of lytic gene expression. ΔRh110 RhCMV/SIV vectors are highly spread deficient in vivo (~1000-fold compared to the parent vector) yet are still able to superinfect RhCMV⁺ RMs and generate high-frequency effector-memory-biased T cell responses. Here, we demonstrate that ΔRh110 68-1 RhCMV/SIV-expressing homologous or heterologous SIV antigens are highly efficacious against intravaginal (IVag) SIV_mac239 challenge, providing control and progressive clearance of SIV infection in 59% of vaccinated RMs. Moreover, among 12 ΔRh110 RhCMV/SIV-vaccinated RMs that controlled and progressively cleared an initial SIV challenge, 9 were able to stringently control a second SIV challenge ~3 years after last vaccination, demonstrating the durability of this vaccine. Thus, ΔRh110 RhCMV/SIV vectors have a safety and efficacy profile that warrants adaptation and clinical evaluation of corresponding HCMV vectors as a prophylactic HIV/AIDS vaccine.

TLR7 agonist administration to SIV-infected macaques receiving early initiated cART does not induce plasma viremia

Reduction/elimination of HIV-1 reservoirs that persist despite combination antiretroviral therapy (cART) will likely require induction of viral expression by residual infected cells and enhanced clearance of these cells. TLR7 agonists have potential to mediate these activities. We evaluated immunologic and virologic effects of repeated doses of the TLR7 agonist GS-9620 in SIV-infected rhesus macaques receiving cART, which was initiated at 13 days after infection and was continued for 75 weeks prior to GS-9620 administration. During cART, GS-9620 induced transient upregulation of IFN-stimulated genes in blood and tissues, increases in plasma cytokines, and changes in immune cell population activation and phenotypes but did not result in measurable increases in plasma viremia or viral RNA-to-viral DNA ratio in PBMCs or tissues nor decreases in viral DNA in PBMC or tissues. SIV-specific CD8+ T cell responses, negligible prior to GS-9620 treatment, were not measurably boosted by treatment; a second course of GS-9620 administration overlapping with later cART discontinuation was associated with increased CD8+ T cell responses during viral recrudescence. These results confirm and extend evidence for GS-9620-mediated enhancement of antiviral immune responses in SIV-infected macaques but suggest that GS-9620-mediated viral induction may depend critically on the timing of initiation and duration of cART and resulting characteristics of viral reservoirs.

Liver macrophage-associated inflammation correlates with SIV burden and is substantially reduced following cART

Liver disease is a leading contributor to morbidity and mortality during HIV infection, despite the use of combination antiretroviral therapy (cART). The precise mechanisms of liver disease during HIV infection are poorly understood partially due to the difficulty in obtaining human liver samples as well as the presence of confounding factors (e.g. hepatitis co-infection, alcohol use). Utilizing the simian immunodeficiency virus (SIV) macaque model, a controlled study was conducted to evaluate the factors associated with liver inflammation and the impact of cART. We observed an increase in hepatic macrophages during untreated SIV infection that was associated with a number of inflammatory and fibrosis mediators (TNFα, CCL3, TGFβ). Moreover, an upregulation in the macrophage chemoattractant factor CCL2 was detected in the livers of SIV-infected macaques that coincided with an increase in the number of activated CD16+ monocyte/macrophages and T cells expressing the cognate receptor CCR2. Expression of Mac387 on monocyte/macrophages further indicated that these cells recently migrated to the liver. The hepatic macrophage and T cell levels strongly correlated with liver SIV DNA levels, and were not associated with the levels of 16S bacterial DNA. Utilizing in situ hybridization, SIV-infected cells were found primarily within portal triads, and were identified as T cells. Microarray analysis identified a strong antiviral transcriptomic signature in the liver during SIV infection. In contrast, macaques treated with cART exhibited lower levels of liver macrophages and had a substantial, but not complete, reduction in their inflammatory profile. In addition, residual SIV DNA and bacteria 16S DNA were detected in the livers during cART, implicating the liver as a site on-going immune activation during antiretroviral therapy. These findings provide mechanistic insights regarding how SIV infection promotes liver inflammation through macrophage recruitment, with implications for in HIV-infected individuals.

Genetically-barcoded SIV facilitates enumeration of rebound variants and estimation of reactivation rates in nonhuman primates following interruption of suppressive antiretroviral therapy

HIV and SIV infection dynamics are commonly investigated by measuring plasma viral loads. However, this total viral load value represents the sum of many individual infection events, which are difficult to independently track using conventional sequencing approaches. To overcome this challenge, we generated a genetically tagged virus stock (SIVmac239M) with a 34-base genetic barcode inserted between the vpx and vpr accessory genes of the infectious molecular clone SIVmac239. Next-generation sequencing of the virus stock identified at least 9,336 individual barcodes, or clonotypes, with an average genetic distance of 7 bases between any two barcodes. In vitro infection of rhesus CD4+ T cells and in vivo infection of rhesus macaques revealed levels of viral replication of SIVmac239M comparable to parental SIVmac239. After intravenous inoculation of 2.2x10⁵ infectious units of SIVmac239M, an average of 1,247 barcodes were identified during acute infection in 26 infected rhesus macaques. Of the barcodes identified in the stock, at least 85.6% actively replicated in at least one animal, and on average each barcode was found in 5 monkeys. Four infected animals were treated with combination antiretroviral therapy (cART) for 82 days starting on day 6 post-infection (study 1). Plasma viremia was reduced from >10⁶ to <15 vRNA copies/mL by the time treatment was interrupted. Virus rapidly rebounded following treatment interruption and between 87 and 136 distinct clonotypes were detected in plasma at peak rebound viremia. This study confirmed that SIVmac239M viremia could be successfully curtailed with cART, and that upon cART discontinuation, rebounding viral variants could be identified and quantified. An additional 6 animals infected with SIVmac239M were treated with cART beginning on day 4 post-infection for 305, 374, or 482 days (study 2). Upon treatment interruption, between 4 and 8 distinct viral clonotypes were detected in each animal at peak rebound viremia. The relative proportions of the rebounding viral clonotypes, spanning a range of 5 logs, were largely preserved over time for each animal. The viral growth rate during recrudescence and the relative abundance of each rebounding clonotype were used to estimate the average frequency of reactivation per animal. Using these parameters, reactivation frequencies were calculated and ranged from 0.33–0.70 events per day, likely representing reactivation from long-lived latently infected cells. The use of SIVmac239M therefore provides a powerful tool to investigate SIV latency and the frequency of viral reactivation after treatment interruption.

Elucidation of HIV dynamics is essential for a thorough understanding of viral transmission, therapeutic interventions, pathogenesis, and immune evasion. The complex dynamics of reservoir establishment and viral recrudescence upon therapy removal present the primary obstacles to developing a functional cure. We sought to develop a virus model system for use in nonhuman primates that allows for the genetic discrimination of nearly 10,000 otherwise isogenic clones. This “synthetic swarm” adds a genetic component to viral dynamics where individual viral lineages can be tracked and monitored during infection. Here we utilized this model to identify the dynamics of viral reservoir establishment and rebound. We found that after 300 or more days of therapy, between 4 and 8 distinct viral lineages could be detected upon therapeutic intervention. Using the relative proportion of each distinct genetic barcoded virus and the overall viral load curve, we could estimate the time and rate of reactivation from latency. On average, we found 1 reactivation event every 2 days with reactivation of the first rebounding variant within days of therapeutic interruption. This virus model will be useful for testing various approaches to reduce the latent viral reservoir and to molecularly track viral dynamics in all stages of infection.

Antibody-mediated protection against SHIV challenge includes systemic clearance of distal virus

HIV-1-specific broadly neutralizing antibodies (bNAbs) can protect rhesus monkeys against simian-human immunodeficiency virus (SHIV) challenge. However, the site of antibody interception of virus and the mechanism of antibody-mediated protection remain unclear. We administered a fully protective dose of the bNAb PGT121 to rhesus monkeys and challenged them intravaginally with SHIV-SF162P3. In PGT121-treated animals, we detected low levels of viral RNA and viral DNA in distal tissues for seven days following challenge. Viral RNA-positive tissues showed transcriptomic changes indicative of innate immune activation, and cells from these tissues initiated infection after adoptive transfer into naïve hosts. These data demonstrate that bNAb-mediated protection against a mucosal virus challenge can involve clearance of infectious virus in distal tissues.

Rapid Inflammasome Activation following Mucosal SIV Infection of Rhesus Monkeys

The earliest events following mucosal HIV-1 infection, prior to measurable viremia, remain poorly understood. Here, by detailed necropsy studies, we show that the virus can rapidly disseminate following mucosal SIV infection of rhesus monkeys and trigger components of the inflammasome, both at the site of inoculation and at early sites of distal virus spread. By 24 hr following inoculation, a proinflammatory signature that lacked antiviral restriction factors was observed in viral RNA-positive tissues. The early innate response included expression of NLRX1, which inhibits antiviral responses, and activation of the TGF-β pathway, which negatively regulates adaptive immune responses. These data suggest a model in which the virus triggers specific host mechanisms that suppress the generation of antiviral innate and adaptive immune responses in the first few days of infection, thus facilitating its own replication. These findings have important implications for the development of vaccines and other strategies to prevent infection.

The ability of TNPO3-depleted cells to inhibit HIV-1 infection requires CPSF6

Background

Expression of the cellular karyopherin TNPO3/transportin-SR2/Tnp3 is necessary for HIV-1 infection. Depletion of TNPO3 expression in mammalian cells inhibits HIV-1 infection after reverse transcription but prior to integration.

Results

This work explores the role of cleavage and polyadenylation specificity factor subunit 6 (CPSF6) in the ability of TNPO3-depleted cells to inhibit HIV-1 infection. Our findings showed that depletion of TNPO3 expression inhibits HIV-1 infection, while the simultaneous depletion of TNPO3 and CPSF6 expression rescues HIV-1 infection. Several experiments to understand the rescue of infectivity by CPSF6 were performed. Our experiments revealed that the HIV-1 capsid binding ability of the endogenously expressed CPSF6 from TNPO3-depleted cells does not change when compared to CPSF6 from wild type cells. In agreement with our previous results, depletion of TNPO3 did not change the nuclear localization of CPSF6. Studies on the formation of 2-LRT circles during HIV-1 infection revealed that TNPO3-depleted cells are impaired in the integration process or exhibit a defect in the formation of 2-LTR circles. To understand whether the cytosolic fraction of CPSF6 is responsible for the inhibition of HIV-1 in TNPO3-depleted cells, we tested the ability of a cytosolic full-length CPSF6 to block HIV-1 infection. These results demonstrated that overexpression of a cytosolic full-length CPSF6 blocks HIV-1 infection at the nuclear import step. Fate of the capsid assays revealed that cytosolic expression of CPSF6 enhances stability of the HIV-1 core during infection.

Conclusions

These results suggested that inhibition of HIV-1 by TNPO3-depleted cells requires CPSF6.

Characterization of human immunodeficiency virus type 1 (HIV-1) containing mutations in the nucleocapsid protein at a putative HIV-1 protease cleavage site

The HIV-1 nucleocapsid protein (NC) has been hypothesized to be cleaved by the viral protease (PR) during early infection. Characterization of viruses, with amino-acid substitutions that modulate PR cleavage of NC in vitro, was performed in cell culture. Two of the NC mutants, NCN17F and NCN17G, had decreased infectivity and exhibited severe H9 replication defects. Examination of viral DNA after infections revealed defects in reverse transcription and integration, although integration defects were cell-type dependent. However, while the defects in reverse transcription and integration correlate with lowered infectivity in a single-round of infection, they did not approach the magnitude of the replication defect measured in H9 cells over multiple rounds. Importantly, we fail to see evidence that H9 cells are re-infected with the NCN17G and NCN17F viruses 24 h after the initial infection, which suggests that the principal defect caused by these NC mutations occurs during late events of viral replication.

Human immunodeficiency virus type 1 nucleocapsid zinc-finger mutations cause defects in reverse transcription and integration

The nucleocapsid (NC) protein from HIV-1 contains two zinc-fingers, both of which are necessary for virus replication. This is the first in-depth study that presents the effects of nucleocapsid zinc-finger substitutions on the kinetics of reverse transcription and integration. Over a 72-h time-course of infection, the quantities of viral DNA (vDNA) observed with viruses containing either the nucleocapsid His23Cys or His44Cys mutations were significantly lower than those observed in infections with virus containing wild-type NC. In addition, the kinetics of vDNA formation and loss were significantly different from wild-type. The kinetic profiles observed indicated reduced vDNA stability, as well as defects in reverse transcription and integration. Overall, the defect in integration was much more pronounced than the reverse transcription defects. This suggests that the principal reason for the replication defectiveness of these mutant viruses is impairment of integration, and thus demonstrates the critical importance of NC in HIV-1 infection.

Human cellular nucleic acid-binding protein Zn2+ fingers support replication of human immunodeficiency virus type 1 when they are substituted in the nucleocapsid protein

A family of cellular nucleic acid binding proteins (CNBPs) contains seven Zn(2+) fingers that have many of the structural characteristics found in retroviral nucleocapsid (NC) Zn(2+) fingers. The sequence of the NH(2)-terminal NC Zn(2+) finger of the pNL4-3 clone of human immunodeficiency virus type 1 (HIV-1) was replaced individually with sequences from each of the seven fingers from human CNBP. Six of the mutants were normal with respect to protein composition and processing, full-length genomic RNA content, and infectivity. One of the mutants, containing the fifth CNBP Zn(2+) finger (CNBP-5) packaged reduced levels of genomic RNA and was defective in infectivity. There appear to be defects in reverse transcription in the CNBP-5 infections. Models of Zn(2+) fingers were constructed by using computational methods based on available structural data, and atom-atom interactions were determined by the hydropathic orthogonal dynamic analysis of the protein method. Defects in the CNBP-5 mutant could possibly be explained, in part, by restrictions of a set of required atom-atom interactions in the CNBP-5 Zn(2+) finger compared to mutant and wild-type Zn(2+) fingers in NC that support replication. The present study shows that six of seven of the Zn(2+) fingers from the CNBP protein can be used as substitutes for the Zn(2+) finger in the NH(2)-terminal position of HIV-1 NC. This has obvious implications in antiviral therapeutics and DNA vaccines employing NC Zn(2+) finger mutants.

Human immunodeficiency virus type 1 nucleocapsid zn(2+) fingers are required for efficient reverse transcription, initial integration processes, and protection of newly synthesized viral DNA

Human immunodeficiency virus type 1 (HIV-1) containing mutations in the nucleocapsid (NC) Zn(2+) finger domains have greatly reduced infectivity, even though genome packaging is largely unaffected in certain cases. To examine replication defects, viral DNA (vDNA) was isolated from cells infected with viruses containing His-to-Cys changes in their Zn(2+) fingers (NC(H23C) and NC(H44C)), an integrase mutant (IN(D116N)), a double mutant (NC(H23C)/IN(D116N)), or wild-type HIV-1. In vitro assays have established potential roles for NC in reverse transcription and integration. In vivo results for these processes were obtained by quantitative PCR, cloning of PCR products, and comparison of the quantity and composition of vDNA generated at discrete points during reverse transcription. Quantitative analysis of the reverse transcription intermediates for these species strongly suggests decreased stability of the DNA produced. Both Zn(2+) finger mutants appear to be defective in DNA synthesis, with the minus- and plus-strand transfer processes being affected while interior portions of the vDNA remain more intact. Sequences obtained from PCR amplification and cloning of 2-LTR circle junction fragments revealed that the NC mutants had a phenotype similar to the IN mutant; removal of the terminal CA dinucleotides necessary for integration of the vDNA is disabled by the NC mutations. Thus, the loss of infectivity in these NC mutants in vivo appears to result from defective reverse transcription and integration processes stemming from decreased protection of the full-length vDNA. Finally, these results indicate that the chaperone activity of NC extends from the management of viral RNA through to the full-length vDNA.

Characterization of the block in replication of nucleocapsid protein zinc finger mutants from moloney murine leukemia virus

Mutagenesis studies have shown that retroviral nucleocapsid (NC) protein Zn(2+) fingers (-Cys-X(2)-Cys-X(4)-His-X(4)-Cys- [CCHC]) perform multiple functions in the virus life cycle. Moloney murine leukemia virus mutants His 34-->Cys (CCCC) and Cys 39-->His (CCHH) were able to package their genomes normally but were replication defective. Thermal dissociation experiments showed that the CCHH mutant was not defective in genomic RNA dimer structure. Primer tRNA placement on the viral genome and the ability of the tRNA to function in reverse transcription initiation in vitro also appear normal. Some "full-length" DNA copies of the viral genome were synthesized in mutant virus-infected cells. The CCCC and CCHH mutants produced these DNA copies at greatly reduced levels. Circle junction fragments, amplified from two-long-terminal-repeat viral DNA (vDNA) by PCR, were cloned and characterized. Remarkably, it was discovered that vDNA isolated from cells infected with mutant virions had a wide variety of abnormalities at the site at which the two ends of the linear precursor had been ligated to form the circle (i.e., the junction between the 5' end of U3 and the 3' end of U5). In some molecules, bases were missing from regions corresponding to the U3 and U5 linear vDNA termini; in others, the viral sequences extended either beyond the U5 sequences into the primer-binding site and 5' leader or beyond the U3 sequences into the polypurine tract into the env coding region. Still other molecules contained nonviral sequences between the linear vDNA termini. Such defective genomes would certainly be unsuitable substrates for integration. Thus, strict conservation of the CCHC structure in NC is required for infection events prior to and possibly including integration.

Strict conservation of the retroviral nucleocapsid protein zinc finger is strongly influenced by its role in viral infection processes: characterization of HIV-1 particles containing mutant nucleocapsid zinc-coordinating sequences

The retroviral nucleocapsid (NC) protein contains highly conserved amino acid sequences (-Cys-X2-Cys-X4-His-X4-Cys-) designated retroviral (CCHC) Zn2+ fingers. The NC protein of murine leukemia viruses contains one NC Zn2+ finger and mutants that were competent in metal binding (CCCC and CCHH) packaged wild-type levels of full-length viral RNA but were not infectious. These studies were extended to human immunodeficiency virus type 1 (HIV-1), a virus with two NC Zn2+ fingers. Viruses with combinations of CCHC, CCCC, and CCHH Zn2+ fingers in each position of HIV-1 NC were characterized. Mutant particles contained the normal complement of processed viral proteins. Four mutants packaged roughly wild-type levels of genomic RNA, whereas the remaining mutants packaged reduced levels. Virions with mutated C-terminal position NC fingers were replication competent. One interesting mutant, containing a CCCC Zn2+ finger in the N-terminal position of NC, packaged wild-type levels of viral RNA and showed approximately 5% wild-type levels of infectivity when examined in CD4-expressing HeLa cells containing an HIV-1 LTR/beta-galactosidase construct. However, this particular mutant was replication defective in H9 cells; all other mutants were replication defective over the 8-week course of the assay. Two long terminal repeat viral DNA species could be detected in the CCCC mutant but not in any of the other replication-defective mutants. These studies show that the N-terminal Zn2+ finger position is more sensitive to alterations than the C-terminal position with respect to replication. Additionally, the retroviral (CCHC) NC Zn2+ finger is required for early infection processes. The evolutionary pressure to maintain CCHC NC Zn2+ fingers depends mainly on its function in infection processes, in addition to its function in genome packaging.

Nucleocapsid protein zinc-finger mutants of simian immunodeficiency virus strain mne produce virions that are replication defective in vitro and in vivo

All retroviruses (except the spumaretroviruses) contain a nucleocapsid (NC) protein that encodes one or two copies of the Zn2+-finger sequence -Cys-X2-Cys-X4-His-X4-Cys-. This region has been shown to be essential for recognition and packaging of the genomic RNA during virion particle assembly. Additionally, this region has been shown to be involved in early infection events in a wide spectrum of retroviruses, including mammalian type C [e.g., murine leukemia virus (MuLV)], human immunodeficiency virus type 1 (HIV-1), Rous sarcoma virus, and other retroviruses. Mutations in the two Zn2+-fingers of the NC protein of simian immunodeficiency virus strain Mne [SIV(Mne)] have been generated. The resulting virions contained the normal complement of processed viral proteins with densities indistinguishable from wild-type SIV(Mne). All of the mutants had electron micrograph morphologies similar to those of immature particles observed in wild-type preparations. RNA packaging was less affected by mutations in the NC protein of SIV(Mne) than has been observed for similar mutants in the MuLV and HIV-1 systems. Nevertheless, in vitro replication of SIV(Mne) NC mutants was impaired to levels comparable to those observed for MuLV and HIV-1 NC mutants; replication defective NC mutants are typically 10(5)- to 10(6)-fold less infectious than similar levels of wild-type virus. One mutant, DeltaCys33-Cys36, was also found to be noninfectious in vivo when mutant virus was administered intravenously to a pig-tailed macaque. NC mutations can therefore be used to generate replication defective virions for candidate vaccines in the SIV macaque model for primate lentiviral diseases.

Wild-type and mutant HIV type 1 nucleocapsid proteins increase the proportion of long cDNA transcripts by viral reverse transcriptase

HIV-1 nucleocapsid, p7, contains two retroviral zinc fingers, which are both necessary for efficient packaging of genomic RNA and infectivity. The nucleocapsid protein is bound tightly to genomic RNA in the mature virion. In this study, the effect of p7 on polymerization of nascent cDNA by viral reverse transcriptase (RT) was examined. An 874-base RNA of HIV-1 was synthesized and used as a template in RT assays with varying concentrations of intact p7, mutants of p7 that have transposed or repeated zinc fingers, and several different peptides that represent various structural regions of p7. Results indicate that at greater than or equal to 50% saturation of p7-binding sites, with p7, there is up to a 90% reduction in total cDNA synthesis, as measured by nucleotide incorporation. However, the cDNA products that are made are almost exclusively full length. Three zinc finger mutants exhibited effects similar to those of wild-type p7. N-terminal and C-terminal halves of p7 inhibited total nucleotide incorporation, but also inhibited synthesis of long cDNA products by RT. In the absence of p7 an array of short transcripts (< 200 bases) was produced by RT. These studies show that full-length p7 is necessary to increase the proportion of long cDNA transcripts produced by RT. The relative position of the two zinc fingers is not critical for this effect.

Microvesicles are a source of contaminating cellular proteins found in purified HIV-1 preparations

Identification and quantitation of cellular proteins associated with HIV-1 particles are complicated by the presence of nonvirion-associated cellular proteins that copurify with virions. Many cellular proteins are associated with nonviral particles that bud from the surface of cells called microvesicles. Microvesicles band in sucrose gradients in a range of densities that includes the same density as retroviruses. To characterize these microvesicles, HIV-1-infected and uninfected human T-cell lines were propagated and virus and microvesicles were purified from clarified cell culture supernatants by sucrose density gradient centrifugation or centrifugation through 20% sucrose pads. Microvesicles were found to contain various proteins, including HLA DR and beta 2-M, and a substantial amount of RNA and DNA. The concentrations of HIV-1 p24CA, HLA DR and beta 2-microglobulin (beta 2-M) were determined by radioimmunoassay. The ratios of HIV-1 p24CA to HLA DR and beta 2-M were found to vary with respect to the HIV-1 isolate, host cell, and other factors. Electron microscopic analysis of microvesicles revealed that they consisted of particles of various sizes and morphologies. Although HIV-1 particles are known to contain some cellular proteins, microvesicles from HIV-1 infected H9 cells appeared to contain little or no HIV-1 gp120SU.

Human immunodeficiency virus type 1 nucleocapsid protein reduces reverse transcriptase pausing at a secondary structure near the murine leukemia virus polypurine tract

In an earlier study on minus-strand DNA synthesis catalyzed by murine leukemia virus reverse transcriptase, we described a prominent pause site near the polypurine tract (J. Guo, W. Wu, Z. Y. Yuan, K. Post, R. J. Crouch, and J. G . Levin, Biochemistry 34:5018-5029, 1995). We now report that pausing at this site is due to a stem-loop structure in the RNA template, formed by interaction of a number of bases in the polypurine tract, including the six G's, and a 3' sequence which includes four C's. Addition of human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein to reverse transcriptase reactions reduces pausing by approximately 8- to 10-fold and stimulates synthesis of full-length DNA. Thus, NC functions as an accessory protein during elongation of minus-strand DNA and increases the efficiency of DNA synthesis, in this case, by apparently destabilizing a region of secondary structure in the template. Since NC is associated with genomic RNA in the viral core and is likely to be part of a viral replication complex, these results suggest that NC may also promote efficient DNA synthesis during virus replication. Mutational analysis indicates that the features of HIV-1 NC which are important for reduction of pausing include the basic amino acids flanking the first zinc finger, the zinc fingers, and the cysteine and aromatic amino acids within the fingers. These findings suggest that reverse transcription might be targeted by drugs which inactivate the zinc fingers of HIV-1 NC.

HIV-1 nucleocapsid protein induces "maturation" of dimeric retroviral RNA in vitro

After a retrovirus particle is released from the cell, the dimeric genomic RNA undergoes a change in conformation. We have previously proposed that this change, termed maturation of the dimer, is due to the action of nucleocapsid (NC) protein on the RNA within the virus particle. We now report that treatment of a 345-base synthetic fragment of Harvey sarcoma virus RNA with recombinant or synthetic HIV-1 NC protein converts a less stable form of dimeric RNA to a more stable form. This phenomenon thus appears to reproduce the maturation of dimeric retroviral RNA in a completely defined system in vitro. To our knowledge, maturation of dimeric RNA within a retrovirus particle is the first example of action of an "RNA chaperone" protein in vivo. Studies with mutant NC proteins suggest that the activity depends upon basic amino acid residues flanking the N-terminal zinc finger and upon residues within the N-terminal finger, including an aromatic amino acid, but do not require the zinc finger structures themselves.

Transforming growth factor-beta isoform expression in insulin-like growth factor stimulated myogenesis

Transforming growth factor betas (TGF-beta s) are the defining members of a super-family of small proteins that are involved in the regulation of development and morphogenesis in a wide array of systems. Previous studies have demonstrated that TGF-beta s both inhibit and, under specialized conditions, induce the differentiation of myoblasts. TGF-beta have been shown to be secreted by mouse C2C12 myoblast cultures undergoing differentiation. Insulin-like growth factors (IGFs) have also been shown to be secreted by myoblasts and to induce myogenesis. This study characterizes the effect of IGF treatment on the expression and secretion of TGF-beta s in the IGF-sensitive L6A1 myoblast line. IGF downregulated the expression of TGF-beta 3 in a concentration-dependent manner at 24 and 48 hours; TGF-beta 1 was not sensitive to IGF treatment at 24 hours but was downregulated by IGFs at 48 hours. This downregulation was mediated by the type 1 IGF receptor and modulated by IGF binding proteins secreted by the myoblasts. Some reexpression of TGF-beta 1 and TGF-beta 3 mRNAs was observed after extensive morphological differentiation had occurred. These results support the hypothesis that IGFs act through the IGF type I receptor as part of a concerted mechanism to modulate expression of the TGF-beta genes, as part of a coordinated set of changes associated with terminal myogenic differentiation.